Attempt to fix Distribute Tuple False Positive bug

mypy/subtypes.py

@@ -185,6 +185,8 @@ def is_subtype(
         # steps we come back to initial call is_subtype(A, B) and immediately return True.
         with pop_on_exit(type_state.get_assumptions(is_proper=False), left, right):
             return _is_subtype(left, right, subtype_context, proper_subtype=False)
+    left = get_proper_type(left)
+    right = get_proper_type(right)
     return _is_subtype(left, right, subtype_context, proper_subtype=False)
 
 
@@ -282,6 +284,21 @@ def is_same_type(
     )
 
 
+# This is a helper function used to check for recursive type of distributed tuple
+def is_structurally_recursive(typ: Type, seen: set[Type] | None = None) -> bool:
+    if seen is None:
+        seen = set()
+    typ = get_proper_type(typ)
+    if typ in seen:
+        return True
+    seen.add(typ)
+    if isinstance(typ, UnionType):
+        return any(is_structurally_recursive(item, seen.copy()) for item in typ.items)
+    if isinstance(typ, TupleType):
+        return any(is_structurally_recursive(item, seen.copy()) for item in typ.items)
+    return False
+
+
 # This is a common entry point for subtyping checks (both proper and non-proper).
 # Never call this private function directly, use the public versions.
 def _is_subtype(
@@ -303,7 +320,30 @@ def _is_subtype(
         # TODO: should we consider all types proper subtypes of UnboundType and/or
         # ErasedType as we do for non-proper subtyping.
         return True
-
+    if isinstance(left, TupleType) and isinstance(right, UnionType):
+        # check only if not recursive type because if recursive type,
+        # test run into maximum recursive depth reached
+        if not is_structurally_recursive(left) and not is_structurally_recursive(right):
+            fallback = left.partial_fallback
+            tuple_items = left.items
+            if hasattr(left, "fallback") and left.fallback is not None:
+                fallback = left.fallback
+            for i in range(len(tuple_items)):
+                uitems = tuple_items[i]
+                uitems_type = get_proper_type(uitems)
+                if isinstance(uitems_type, UnionType):
+                    new_tuples = [
+                        TupleType(
+                            tuple_items[:i] + [uitem] + tuple_items[i + 1 :], fallback=fallback
+                        )
+                        for uitem in uitems_type.items
+                    ]
+                    result = [
+                        _is_subtype(t, right, subtype_context, proper_subtype=False)
+                        for t in new_tuples
+                    ]
+                    inverted_list = [not item for item in result]
+                    return not any(inverted_list)
     if isinstance(right, UnionType) and not isinstance(left, UnionType):
         # Normally, when 'left' is not itself a union, the only way
         # 'left' can be a subtype of the union 'right' is if it is a

test-data/unit/check-tuples.test

@@ -1041,6 +1041,47 @@ reveal_type(x)  # N: Revealed type is "Tuple[builtins.int, fallback=__main__.Tes
 [out]
 
 
+-- Union-in-Tuple distribution
+-- ---------------------------------------------------------
+
+
+[case testTupleUnionDistributionSuccess]
+from typing import Tuple, Optional, Union
+
+def f1(x: Tuple[float, Optional[float]]) -> Union[Tuple[float, float], Tuple[float, None]]:
+    return x
+
+def f2(x: Tuple[Union[int, str], float]) -> Union[Tuple[int, float], Tuple[str, float]]:
+    return x
+
+def f3(x: Tuple[int, Union[str, None]]) -> Union[Tuple[int, str], Tuple[int, None]]:
+    return x
+
+def f4(x: Tuple[Union[int, float]]) -> Union[Tuple[int], Tuple[float]]:
+    return x
+
+def f5(x: Tuple[Union[int, str], Union[bool, None]]) -> Union[
+    Tuple[int, bool],
+    Tuple[int, None],
+    Tuple[str, bool],
+    Tuple[str, None],
+]:
+    return x
+[builtins fixtures/tuple.pyi]
+[out]
+
+[case testTupleUnionDistributionFail]
+from typing import Tuple, Optional, Union
+
+def g1(x: Tuple[float, Optional[float]]) -> Union[Tuple[float, float], Tuple[str, float]]:
+    return x  # E: Incompatible return value type (got "Tuple[float, Optional[float]]", expected "Union[Tuple[float, float], Tuple[str, float]]")
+
+def g2(x: Tuple[float, Optional[float]]) -> Union[Tuple[float, str], Tuple[float, None]]:
+    return x  # E: Incompatible return value type (got "Tuple[float, Optional[float]]", expected "Union[Tuple[float, str], Tuple[float, None]]")
+[builtins fixtures/tuple.pyi]
+[out]
+
+
 -- Variable-length tuples (Tuple[t, ...] with literal '...')
 -- ---------------------------------------------------------